B-lineage acute lymphoblastic leukemia (ALL) is the most common form of cancer in children. The leukemic lymphoblasts are clonal expansions of B-cell progenitors that grow within the hematopoietic microenvironment. Such cells and their normal counterparts rapidly die by apoptosis in vitro unless they are supported by stromal layers. Under previous funding, culture techniques suitable for maintaining immature B cells and other hematopoietic cells were developed, and molecules involved in the interaction between immature B cells and stroma identified. Stroma-supported cultures of leukemic B-cell precursors have made it possible to measure the growth potential of such cells, their propensity to undergo apoptosis and their sensitivity to anticancer drugs. Our long-term objective is to define the precise microenvironmental requirements for survival, growth and differentiation of normal and leukemic B-cell progenitors and to apply this information to the development of novel strategies of ALL treatment. Cellular components of the microenvironment may have different capacities for supporting the survival and expansion of immature lymphoid cells. In Specific Aim 1, immortal clonal stromal cell lines will be developed from different sites of lymphohematopoiesis (human bone marrow and murine aorta-gonad mesonephros region). The cell lines' ability to support immature B cells and other hematopoietic cells in vitro will then be characterized, and their lineage-association determined. Specific Aim 2 will address the mechanisms by which stromal cells support immature lymphoid cell survival. These studies will build on recent reports and preliminary findings to determine the importance of direct communication between lymphoid and stromal cells and the role of gap junctions in this interaction. Gene profiling studies will be used to determine whether expression of growth factors, adhesion molecules and other potentially important molecules is associated with the ability of stromal cell lines to support lymphoid cells. Studies in Specific Aim 3 will use stroma-supported cultures of primary leukemic cells to conduct preclinical testing of BAY 36-1677, a genetically engineered variant of IL-4 that has unique receptor-reactivity and signaling properties. In preliminary studies, BAY 36-1677 showed powerful and selective cytotoxicity against ALL cells but did not suppress the growth of normal hematopoietic cells and did not affect fibroblasts or endothelial cells. The proposed studies will further characterize the antileukemic activity of BAY 36-1677 and identify synergistically interacting compounds.